To select an appartment door for quick and quiet forced entry a theif will generally push the top and bottom righthand corner of a door as you face it from the outside. A firm push in these two areas will tell him if the door is locked, i.e closed and keys turned in the lock to extend the heavy duty bolts located at the top middle and bottom of the door or if it has just been pulled closed and the latched but not locked.
With the bolts engaged by correctly locking the door it would take considerable time and effort , not to nention noise to open the door. If the door is only latched it takes around 15 seconds with a flexible paint scraper and hardly makes any noise.
One good way to avoid being targetted is to ensure that the door is closed and locked with the key. the dor is not locked if you just close it.